Swift J151857.0-572147: Swift detection of a new galactic X-ray transient
ATel #16500; J. A. Kennea(PSU), A. Y. Lien (U Tampa), V. D'Elia (SSDC & INAF-OAR), A. Melandri (INAF-OAR), K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team
on 3 Mar 2024; 16:38 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)
Subjects: X-ray, Transient
Swift Trigger #1218452 (GCN #35849, 35835), previously named GRB 240303A but considered likely to be a galactic transient named Swift J151857.0-572147, has been confirmed to be a transient. Repeated observations of the source show it to be consistently bright, with no signs of fading. This combined with its localization within the Galactic Plane, and the fact that this was a BAT Image trigger, confirms its nature as a Galactic Transient.
The best localization of the source (as excerpted from GCN #35849) was from the XRT immediate on-board localization, and was found to be RA, Dec 229.7375, -57.3633 which is equivalent to:
RA(J2000) = 15h 18m 57.00s
Dec(J2000) = -57d 21' 47.9"
with an uncertainty of 4.8 arcseconds (radius, 90% containment). We note that onboard localizations may be inaccurate, so follow-up observations will be needed to obtain a more accurate localization. Currently the source is bright and being observed in Windowed Timing mode.
A spectral fit to the combined Swift/XRT data show an absorption N_H = 5.6 +/- 0.06 x 10^22 cm^-2 and a power-law slope with photon index of 1.78 +/- 0.02. The average flux, corrected for absorption is 1.40 +/- 0.02 x 10^-8 erg/s/cm^2 (0.5-10 keV).
We encourage follow-up observations in order to determine the nature of this new transient.
